This invention relates to a watthour meter for very low voltages.
In analog-digital watthour meters precise linearity is required for the conversion of the sensor circuit output voltage into frequency. This requirement appears to be particularly compelling at very low output voltages of the Hall sensor and, additionally, in a wide measuring range.
In certain prior art voltage-frequency converters the influence of the offset voltage of the integrating operational amplifier is reduced by reversing the sign of the input voltage of the integrator, and the influence of the integrating capacitor is removed by applying an accurately determined compensation impulse. The conversion inaccuracy originating from the input voltage of said amplifier remains because of unequal periods of the application of differently polarized voltages. Consequently, such a converter is not suitable for low input voltages, especially for a wide measuring range. A further disadvantage consists in a considerable influence of the integrator loss angle when the amplification of the operational amplifier is restricted.